SPR accuracy poor- ideas?

I built an SPR out of my DPMS and can't get it to shoot anywhere near how it should. I wonder if anyone has some insight? Installed an 18" barrel from Ranier Arms made on a Rock Creek blank. Wylde chamber, 8 twist, cut rifled, hand lapped. Rifle length gas with clamp-on block. Decent free float ATI tube. Lapped the receiver. 2 stage trigger.
Leupold Mark AR scope in Burris 6-screw rings. Receivers and BCG are stock DPMS.
Ammo- handloads with 69 gr MatchKings and RL-15 in Win brass headspaced to 4/1000, primer pockets uniformed, trimmed, and sorted to +- 0.5gr. Also tried 77s with Varget in Lake City.
I'm getting 3" groups with anything I put in it, either suppressed or with the flash hider.
I'm going to try a different scope but I doubt Leupold let a bad one through. Next will be to disassemble it, inspect everything and reassemble.
I had a load that shot 1/2 inch with 55gr FMJ through the stock barrel- I'm a little disappointed.
Any ideas? Could it be the BCG with 3,000 rounds mated to the new barrel?

 

I have seen Black Hills Match ammo fix accuracy problems.

Another was fixed by having the muzzle crown done. Seems the factory forgot to do it.

 

I don't know what the barrel but torque is. It's aluminum ATI proprietary. I tightened it pretty tight twice, then "tight enough" until the gas tube port timed.

 

The fired cases measure .006 over spec measured on a Wilson gauge. I bump them back .004, so yes technically it's out of spec- but my understanding of headspace is that this is perfectly safe.

I did reuse the bolt, because it's been extremely reliable. I'm wondering if wear on the lugs is giving me play against the receiver extension- but accuracy was great with the stock barrel. How do you check for sloppy bolt fit like that?

 

How old is that barrel?

I know Ranier had a bad batch of 18 inch barrels a while back. My buddy got one and had the same problem as you. He sent it back and his new one is consistently 3/4 MOA with match factory ammo.

 

I got it in July or August

 

Hmmm, I would get a head spaced bolt to start, that could cause issues. When you installed the barrel, before the nut, how tight a fit was it to the action?

 

Something is drastically amiss. Talking about bolt head spacing is generally not a 2" conversation. Same for barrel nut torque, out of hundreds of AR's I have built and rebuilt, I can't recall a single one which I could say barrel torque meant the difference between a 3" group and a 1/2" one. When using a used bolt (or any bolt), I confirm intimate contact by lapping the lugs, but I've never even heard of headspace matching a bolt bringing a 3" rifle down to 1/2".

And yes, your reloading practice of bumping to fit your chamber generally works, although you may want to slow close on a few to ensure it's really fitting properly. Semi-autos can allow more shoulder growth than a bolt gun, so your 6thou growth from sized to firing might not actually represent your chamber length. As long as it closes fully when you slow close it, and you're not relying upon the force of the action to resize the case in the chamber, what you're doing is fine.

Bad scope or shifting mounts would be my first test, then I'd be sending the barrel back. If you are truly capable of 1/2" groups, and your reloads are capable of such, then I can't say I'd waste my time tweaking nooks and crannies to try to squeeze a barrel down from 3" to 1/2" - it's a bridge too far. Good barrels will shoot well with almost anything; what makes a bad barrel bad to me is not shooting well with everything, let alone not shooting well with any thing (good with all vs. good with one).

I don't even really want to ask since it really can't be a seating depth issue... But how far are you jumping those SMK's?

Swap the optic, check the mount, then send that tomato stake back to Rainer.

 

Here's what I found out today. I measured the groups from yesterday, 3" was an exaggeration. They're 2 1/4.
I put a VX1 on and shot right around an inch suppressed. With bare threads it shot 0.725. My thoughts- the Mark AR scope has a very heavy reticle, it su tends almost an inch so I use the 2" target spots. With the VX1's thinner wire, I can use 1" targets. Aim small, miss small.
My plan is to put the Mark AR back on, and being mindful of the above shoot a VERY careful group. If it's close to acceptable, take down the rifle, inspect everything, loctite the barrel to the receiver, torque the nut this time, and if I can afford it upgrade the bolt.
Happy discovery by the way... My receiver trying tool is exactly the right diameter to lap scope rings!